This invention is directed to a specimen prealigning apparatus and method and, more particularly, to an edge gripping semiconductor wafer prealigner that substantially reduces wafer backside damage and particulate contamination.
Integrated circuits are produced from wafers of semiconductor material. The wafers are typically housed in a cassette having a plurality of closely spaced slots, each of which can contain a wafer. The cassette is typically moved to a processing station where the wafers are removed from the cassette, placed in a predetermined orientation (prealigned), and returned to another location for further wafer processing.
Various types of wafer handling devices are known for transporting the wafers to and from the cassette and among processing stations. Many employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The end of the robotic arm typically employs vacuum pressure to releasably hold the wafer to the end of the arm. The robotic arm enters the cassette through the narrow gap between an adjacent pair of wafer slots and engages the backside of a wafer to retrieve it from the cassette. After the wafer has been processed, the robotic arm inserts the wafer back into the cassette.
U.S. Pat. No. 5,513,948 for UNIVERSAL SPECIMEN PREALIGNER, which is assigned to the assignee of this application, and U.S. Pat. No. 5,238,354 for SEMICONDUCTOR OBJECT PRE-ALIGNING APPARATUS describe prior semiconductor wafer prealigners that include a rotating vacuum chuck on which the wafer is placed by a robot arm for prealigning.
Unfortunately, transferring the wafer among the cassette, robot arm, and prealigner may cause backside damage thereto and contamination of the other wafers housed in the cassette because engagement with the wafer may dislodge particles that can fall and settle onto the other wafers. Robotic arms and prealigners that employ a vacuum pressure to grip the wafer can be designed to minimize particle creation. Even the few particles created with vacuum pressure gripping or any other non-edge gripping method are sufficient to contaminate adjacent wafers housed in the cassette. Reducing such contamination is particularly important to maintaining wafer processing yields. Moreover, the wafer being transferred may be scratched or abraded on its backside, resulting in wafer processing damage.
What is needed, therefore, is a wafer gripping technique that can securely, quickly, and accurately prealign wafers while minimizing particle contamination and wafer scratching.
An object of this invention is, therefore, to provide an apparatus and a method for prealigning semiconductor wafers.
Another object of this invention is to provide an apparatus and a method for quickly and accurately prealigning specimens.
A further object of this invention is to provide an apparatus and a method for prealigning wafers while minimizing particle contamination and wafer scratching.
Specimen edge-gripping prealigners of this invention grasp a wafer by at least three edge-gripping capstans that are preferably equally spaced around the periphery of the wafer. Each of the edge-gripping capstans is coupled by a continuous synchronous belt to an axially centered, grooved drive hub that is rotated by a drive motor. Each of the capstans is also coaxially connected to a grooved drive drum that is coupled to the drive hub by one of the continuous synchronous belts, and each belt is routed in a unique location in a set of grooves in the drive drums and the drive hub. The continuous synchronous belts are tensioned by idler pulleys that are mounted to axially rotatable idler plates that are coupled together for common rotation by a belt tensioning motor or some other form of rotary biasing force, such as a spring, solenoid, or vacuum pressure actuated piston.
The edge-gripping capstans and the grooved drive drums are mounted to hinged bearing housings that are pivotally spring biased to preload the grooved drive drums radially away from the axially centered drive hub. The edge-gripping capstans can be driven radially inward to grip the wafer by rotating the belt tensioning motor to apply sufficient tension to overcome the spring preload force on the idler plates. Once gripped, the wafer can be rotated by energizing the drive motor to rotate the drive hub, which rotation is coupled through the tensioned belts and drive drums to the capstans.
The edge-gripping specimen prealigner of this invention is suitable for prealigning semiconductor wafers. Simultaneously rotating all the edge-gripping capstans provides positive rotation of the wafer without wafer surface contact, which eliminates wafer backside damage. Synchronously driving of all the capstans prevents slippage between each capstan and the wafer and thereby results in minimized edge contamination.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.